SVGHMI: Fix browser side exception when some widget are not used, and are then discarded and not present in final SVG. In that case JS code was still making reference to discarded widget elements and was raising exception at init.

          <xsd:attribute name="Command" type="xsd:string" use="optional" default="make -C %(buildpath)s all BEREMIZSRC=%(src)s BEREMIZCFLAGS=%(cflags)s MD5=%(md5)s USE_BEREMIZ=1 FROM_BEREMIZ=1"/>